The MALDI mass spectrometry is an analytical method in which a mixture of a matrix and a polymer is irradiated with laser beam to vaporize and ionize the polymer together with the matrix, thereby measuring a molecular weight of the polymer. A role of the matrix in the MALDI mass spectrometry includes to transmit energy of the laser beam to the polymer, thereby ionizing the polymer. Therefore, affinity between the matrix and the polymer is important. As a guide of the affinity, it has been recommended to use a highly polar matrix, for example, α-cyano-4-hydroxycinnamic acid (CHCA) or 2,5-dihydroxybenzoic acid (DHB) for the measurement of a highly polar polymer, for example, polyethylene glycol (PEG). Further, it has been recommended to use a low polar matrix, for example, dithranol or retinoic acid (RA) for the measurement of a low polar polymer, for example, polystyrene (PS) (Non-Patent Documents 1 and 2). Moreover, it has been reported that in the MALDI mass spectrometry of polyethylene glycol having a molecular weight of 10,000, CHCA which is the highly polar matrix is the best (Non-Patent Document 3).
On the other hand, it is generally known that the measurement of a high molecular weight polymer by the MALDI mass spectrometry is difficult. This is because that since the ionization becomes difficult with increase in a molecular weight, it is necessary to increase the laser intensity irradiated, but decomposition of the polymer molecule by the laser irradiation tends to occur. Therefore, by way of example, in the case of polyethylene glycol, only the measurement results for the polyethylene glycols having a molecular weight of 25,000 or less are reported (Non-Patent Document 2).